Restraint-type differential relay

Abstract

A differential relay for protecting a power apparatus portion of a power system, such as a bus line, includes incoming and outgoing power lines on which incoming and outgoing currents are present. The incoming and outgoing currents of each phase are applied through current transformers and then through filtering and normalizing circuits prior to application to the differential relay. The differential relay includes a circuit for determining the presence of an external fault, and maintains security by avoiding a tripping action of the relay for said external faults. Further, the relay includes circuits which in response to the external fault evolving to an internal fault alters the operation of a portion of the relay, by increasing the gain for the restraint current, enabling a directional element for supervision of the differential relay, and for extending the immediate post-fault value of restraint current, and adding a time delay for a selected period of time, all of which are used to identify an external fault evolving into an internal fault.

What is claimed is:  
     
       1. A differential relay for protecting a power system apparatus in a power system, the power system apparatus having a power line input and a power line output, comprising: 
       means for detecting the power signal current present on the power line input and the power line output, respectively;  
       differential element means for recognizing a fault condition in the power system external to the power system apparatus;  
       means for preventing a tripping signal in response to the external fault condition without blocking the differential element; and  
       means for recognizing an internal fault, including the evolution of said external fault into an internal fault, and for permitting a tripping action for the internal fault, from the time of recognition of the external fault, without any preestablished time delay.  
     
     
       2. The relay of  claim 1 , wherein the recognizing means includes means for changing the processing of the power signal currents from the input and output power lines in response to the recognition of an external fault. 
     
     
       3. The relay of  claim 1 , including current transformers for reducing the magnitude of the input and output signal currents to a level suitable for processing by said relay, the relay further including means for filtering and normalizing the reduced current values from the current transformers. 
     
     
       4. The relay of  claim 3 , wherein the filtering means includes a series connection of an analog low pass filter and a digital band pass filter. 
     
     
       5. The relay of  claim 1 , wherein said external fault recognizing means includes a control circuit responsive to values of operating current and restrain current, wherein the input signal current is I 1  and the output signal current is I 2  and wherein operating current is |I 1 +I 2 | and restraint current is |I 1 |+|I 2 |, and wherein an output signal from the control circuit indicating an external fault condition is applied to at least one of three following elements: (a) a memory element which, in response to the output signal from the control circuit, extends the value of the restraint current for a selected period of time; (b) a gain control circuit which, in response to the signal from the control circuit, increases the gain for the restraint current to produce a revised current value; and (c) a directional element which, in response to the signal from the control circuit, determines whether a fault condition recognized by the relay is an external or internal fault. 
     
     
       6. A relay of  claim 5 , wherein the signal from the control circuit is applied to all three elements. 
     
     
       7. A relay of  claim 5 , including means for producing a first comparator output signal when the operating current value is greater than the revised restraint current value and for producing a second comparator output signal when the operating current is greater than a first threshold value, wherein the relay produces a trip signal when the first and second comparator signals are high. 
     
     
       8. A relay of  claim 7 , including means for comparing an incremental value of restraint current against a second threshold value and for enabling the relay to produce a trip signal during a selected period of time if the incremental value is greater than the second threshold value. 
     
     
       9. A relay of  claim 8 , including a switch which selectively enables said incremental value comparing means. 
     
     
       10. A relay of  claim 6 , including means for producing a first comparator output when the operating current value is greater than the revised restraint current value and for producing a second comparator output signal when the operating current is greater than a a first threshold value and for producing a directional element output signal indicative of the internal fault when an internal fault occurs, wherein the relay produces a trip signal when the first and second comparator signals and the directional element output signal are all high. 
     
     
       11. A relay of  claim 10 , including means for comparing the incremental value of restraint current against a second threshold value and for enabling the relay to produce a trip signal during a selected period of time if the incremental value is greater than the second threshold. 
     
     
       12. A relay of  claim 1 , wherein the current values are phase values. 
     
     
       13. A relay of  claim 1 , wherein the current values are RMS (root mean square) values. 
     
     
       14. A relay of  claim 1 , wherein the current values are instantaneous values. 
     
     
       15. A relay of  claim 1 , including a timer means for delaying any trip signal output of the relay.